1. Field of the Invention
The present invention pertains to systems for and methods of measuring and inspecting the patterned features on lithographic photomasks such as the metal patterns which are used in printing the surface patterns on semiconductor chips.
2. Description of the Prior Art
The ever increasing demand for greater information or processing capacity on a smaller area of a silicon wafer has naturally resulted in smaller and smaller circuitry patterns on the chips. This, in turn, has required inspection microscopes of greater and greater resolution so that the dimensions and alignment of the lines on the patterns of the microchips can be accurately checked and controlled. It has recently been felt that further decreases in microchip circuitry size can be achieved by putting greater efforts into the inspection and careful control of the formation of the metallic patterns of the photomasks, which patterns are projected in the conventional optical lithography process onto the silicon wafers. Thus, the inspection and control of the process for forming the patterns of the photomask has recently received a great deal of interest.
In inspecting or measuring the dimensions of patterns on photomasks by conventional optical microscopes there is a problem due to the photoresist or other material which may overlie and hence obscure the etched, partially etched or unetched metal patterns in various photomasks and during various photomask manufacturing processes. The foregoing problem has been exacerbated by the development of iterative etching processes whereby the metal is etched away beneath the photoresist layer in a series of steps with inspection being performed by the microscope between steps in order to improve the quality, i.e., the accuracy of the finished product.
In prior art systems for microscopically inspecting photomasks, transmission microscopes were generally used, i.e., optical systems wherein the light source is directed at the quartz unpatterned face of the mask and the microscope is directed at the opposite, patterned face of the mask to detect the light transmitted through the etched areas. However, if photoresist, anti-reflective material or other material overlies the metal pattern on the mask, the transmitted light is distorted thereby making it quite difficult for the systems to accurately determine the location of the edges of the metal. As the degree of the undercut of the metal beneath the photoresist increases, as in iterative etching processes, the resulting loss of accuracy quite obviously limits the ability of the systems to make the necessary measurements of the small dimensions and hence apply the correct processing to achieve the correct final product.
Other prior art inspection systems using reflected light from the patterned face of the mask, rather than transmitted light through the mask, suffer the same problems. For example, where light is directed from a microscope to a small spot on the face of the photomask and reflected back through the microscope to accurately determine the location of the edges of the metal, the overlying photoresist during the iterative etching processes makes the relevant edges of the patterned features hard to define.
Other types of photomasks are currently being developed which present problems using the conventional microscope inspection technique. For example, current phase-shift lithography may utilize masks with multiple-patterned layers of materials of varying thicknesses or with varying patterned and substrate thicknesses. In conventional inspection techniques, wherein the imaging is done from the patterned side of the mask, accurate measurements are difficult to obtain.